1. Field of the Invention
This invention relates to modified propylene polymer compositions of improved impact resistance. More particularly, the invention relates to blends of sequentially polymerized propylene copolymers with linear low density ethylene copolymers.
2. Description of the Prior Art
Polypropylene is a well known commercial polymer, used for a variety of products such as packaging films and extruded and molded shapes. It is produced by polymerization of propylene over transition metal coordination catalysts, specifically titanium halide containing catalysts. Commercial polypropylene is deficient in resistance to impact at low temperatures, i.e., 0.degree. C. and below. It is known that incorporation of some elastomers, particularly elastomeric copolymers of ethylene and propylene, improves the low temperature impact resistance of polypropylene.
One method of incorporating elastomeric ethylene-propylene copolymers into polypropylene is by sequential polymerization of propylene and ethylene-propylene mixtures. In typical processes of this kind, propylene homopolymer is formed in one stage and the copolymer is formed in a separate stage, in the presence of the homopolymer and of the original catalyst. Multiple stage processes of this type are also known. Products of such sequential polymerization processes are often referred to as "block copolymers" but it is now understood that such products may rather be intimate blends of polypropylene and ethylenepropylene elastomer. The products of such sequential polymerization of propylene and ethylene-propylene mixtures, are referred to herein as sequentially polymerized propylene-ethylene copolymers or as in-situ produced copolymers. To maintain separate terminology for the total sequentially polymerized copolymer composition and the elastomeric copolymer fraction thereof, I may refer to the total copolymer composition as impact-improved propylene-ethylene copolymer which has a specified content of an elastomeric ethylene-propylene copolymer fraction and which is the product of sequential polymerization of propylene and a propylene-ethylene mixture.
Methods for producing impact-improved, sequentially polymerized propylene-ethylene copolymers are well known. See, for example, "Toughened Plastics" by C. B. Bucknall, Applied Science Publishers Ltd. 1977, pp 87-90, and T. G. Heggs in Block Copolymers, D. C. Allport and W. H. James (eds), Applied Science Publishers Ltd. 1973, chapter 4. Representative U.S. patents describing such methods are: U.S. Pat. Nos. 3,200,173--Schilling; 3,318,976--Short; and 3,514,501--Leibson et al.
Polyethylene is generally not compatible with polypropylene and not broadly useful for inclusion in propylene compositions to provide impact improvement. However, some specific compositions of polypropylene and polyethylene, often with other components, have been taught, as discussed below.
The art recognizes three types of polyethylene:
"High density polyethylenes" (HDPE), typically having densities in the range of 0.941 to 0.965 g/cc, may be produced by means of transition metal catalysts of the Ziegler-Natta type or Phillips Petroleum Company's chromia type in processes operating at relatively low pressures. They may also be referred to as low pressure polyethylenes. HDPEs are characterized by linearity and crystallinity.
"Low density polyethylenes" (LDPE), typically having densities in the range of 0.91 to 0.925 g/cc, are produced by a free radical mechanism in equipment operating at very high pressure. They are also referred to as high pressure polyethylene. Conventional LDPE is characterized by branching and absence of crystallinity.
"Linear low density polyethylenes" (LLDPE) are copolymers of ethylene with up to 15% of higher alpha olefins which are the products of polymerization at low pressures over certain transition metal catalysts. The products generally have densities in the range of 0.91 to 0.94 g/cc. A typical vapor phase process for the production of LLDPE is described in U.S. Pat. No. 4,011,382--Levine et al. Properties of LLDPE polymers produced by said process are also given in U.S. Pat. Nos. 4,243,619--Fraser et al and 4,303,710--Bullard et al. Liquid phase processes and their products are described in U.S. Pat. No. 4,128,607 to Ito et al and in U.K. Pat. No. 1,543,908 to Imperial Chemical Industries. LLDPEs are also described in brochures available from manufacturers of such polymers, including Dow Chemical Company, Exxon Corporation and Union Carbide Corporation.
LLDPE is characterized by linearity, but with regular short branches due to inclusion of the co-monomer. It has a low crystallinity and, hence, a lower density than linear HDPE and is not interchangeable with HDPE in typical HDPE applications. The differences between the properties of conventional LDPE produced in high pressure processes and LLDPE are such that the products cannot be used interchangeably in many applications and in many methods of manufacturing articles.
Compositions of polypropylene with various types of polyethylenes are disclosed in the following patents:
The use of LDPE in polypropylene to provide film compositions, which may be oriented polypropylene film compositions of improved impact resistance, are disclosed in British Pat. Nos. 1,005,333 to Avisun and 1,139,887 to Union Carbide Corporation, French Pat. No. 1,562,860 to Ethylene Plastique and German published applications Nos. 1,569,429 to VebaChemie and 2,042,342 to Lentia. Use of LDPE in propylene polyallomers to provide coating compositions is disclosed in U.S. Pat. No. 3,887,640 to Diaz et al.
Whereas the above patents employ LDPE in polypropylene film compositions, several patents directed to polypropylene compositions for other uses, i.e., for shaped articles, employ HDPE. These are U.S. Pat. Nos. 3,281,501 to Coats et al; 3,358,053 to Hostetler; 3,627,852 to Aishima; 3,647,922 to Leugering et al; 3,929,932 and 3,937,758 both to Castagna et al. Coats et al, disclose compositions of polypropylene with "low pressure linear polyethylene" (HDPE) as having improved impact resistance and state that the high pressure process, branched polyethylenes are not effective for the desired impact improvement. Hostetler is directed to compositions in which the impact resistance of propylene/ethylene copolymer is improved by addition of polyethylene which is a substantially linear polymer of 0.93-0.96 density, produced by a low pressure catalyst system (HDPE), and milling of the blend at 175.degree.-225.degree. C. for 7-15 minutes. The propylene/ethylene copolymer of Hostetler may be a block copolymer or a random copolymer. Aishima is directed to ternary compositions consisting of polypropylene, polyethylene and ethylene/propylene block copolymer having an ethylene content of 7-93 mole percent. It is asserted that these compositions have enhanced impact resistance at low temperatures without loss of rigidity, strength and resistance to high temperature. Polyethylene which may be used can be either HDPE or LDPE; however, the use of the high density polyethylene is preferred. The Castagna et al patents are direct to compositions of sequentially polymerized propylene-ethylene copolymers with linear polyethylenes having a relatively high density, i.e., a density of at least 0.93 and a melt index in the range of about 0.2 to 1.0. The polyethylene is disclosed as being known and described in three U.S. patents which are directed to use of supported chromia catalysts, and as being a homopolymer or copolymer with up to 5% of a C.sub.3 -C.sub.8 comonomer. The polyethylenes used in the illustrative examples were homopolymers. Leugering et al blend polyethylenes of melt index 1-30, produced by low pressure polymerization over transition metal catalysts, with sequentially polymerized propylene-ethylene copolymers to produce molding compositions of good low temperature impact resistence and improved transparency.
U.S. Pat. No. 4,172,875 to Beijen et al, is directed to forming of articles from modified polypropylene sheet by fluid pressure forming in the solid phase. Improved articles are produced by employing sheet made of a composition consisting of polypropylene or poly(propylene/ethylene) with low density polyethylene.
In the production of articles of substantial thickness, e.g., 2 mm or more, from polypropylene, one of the desirable properties is stiffness. In articles which may be exposed to low temperature during storage or use, another desired property is impact resistance at low temperature. As explained above, improved impact resistance is conventionally achieved by incorporation of ethylene-propylene elastomer copolymer, either by blending or by sequential polymerization. In such compositions there is invariably a tradeoff between stiffness and low temperature impact resistance, since incorporation of elastomer decreases stiffness. This invention is directed to compositions which provide a desired balance of these three properties in an economical manner.